The present invention relates to an electrical machine having a housing, a first bearing device, which is in the form of a movable bearing, a second bearing device, which is in the form of a fixed bearing, and a shaft, which is mounted with the two bearing devices in the housing such that it can rotate.
The radial resilience of rolling bearings has a substantial influence on the operational response of machines since, owing to the additional elasticity of the bearings, the bending-critical speeds of the rotors are reduced in comparison with the ideal rigid mounting. Rolling bearings also generally have a lower rigidity in the horizontal transverse direction than in the vertical transverse direction since the radial prestress there is not provided by the rotor weight. This results in the natural bending forms being split into a horizontal and a vertical natural bending form, in which case the horizontal one is lower in terms of frequency and therefore oscillation and noise problems are to be expected even at lower speeds.
In practice, this problem is solved in a known manner by rolling bearings being used which can be axially prestressed, for example deep groove ball bearings or angular ball bearings. Above all, the horizontal transverse rigidity of the bearings and therefore also the possible speed range of the machine are therefore increased. In addition, the noise response and the service life of the bearings are positively influenced.
DE 196 54 089 A1 has disclosed a deep groove ball bearing, in the case of which axial prestress is achieved within the bearing by the rolling bodies being pushed alternately axially into one and the opposite end position of the running surfaces by construction elements of the cage. Accordingly, the bearing used is not a bearing which is available as standard. DE 689 04 444 T2 and DE 36 26 626 C2 also describe special constructions of rolling bearings having the option of axial prestress. These variants likewise have the disadvantage that the solutions exclude standard components.
DE 42 24 980 A1 describes a design solution which uses standard rolling bearings and prestresses the bearings in one direction so severely that external loads can bring about a fluctuation in the value of the axial prestress but cannot bring about a reversal of direction. This approach solves the problem in principle, but high static loads on the bearings are even produced within the machine which reduce the service life.
DE 30 38 112 A1 describes a construction with an axial prestress of the bearings which can be adjusted and readjusted. In this case too, some special elements are required and the inner ring of a bearing needs to be designed to be displaceable on the shaft. Since the inner ring in many cases has circumferential load, there is the risk with this variant of frictional corrosion.
In the case of a floating mounting, which is generally known, the two bearings are prestressed, but the rotor is not mounted in a statically determined manner, i.e. it has a degree of translatory freedom in the axial direction.